How to use the viewer node?
You either need to have the backdrop enabled or in the UV/Image Editor set to 'Viewer Node'



Form the manual:

The Viewer node is a temporary, in-process viewer. It could be plug in anywhere to inspect an image or value map in your node tree.

Select a view node with LMB to switch between multiple view nodes. It is possible to automatically plug a Viewer node to any other node by pressing Shift-Ctrl-LMB on it.

Using the UV/Image Editor

The viewer node allows results to be displayed in the UV/Image Editor. The image is facilitated in the header by selecting Viewer Node in the linked Image data-block menu. The UV/Image Editor will display the image from the currently selected viewer node.

To save the image being viewed, use Image ‣ Save As Image, F3 to save the image in a file.

The UV/Image Editor also has three additional options in its header to view Images with or without Alpha, or to view the Alpha or Z itself. Holding LMB in the Image display allows you to sample the values.